High melt flow fluoropolymer

ABSTRACT

The present invention relates to a partially-crystalline copolymer comprising tetrafluoroethylene, hexafluoropropylene in an amount corresponding to a hexafluoropropylene index (HFPI) of from about 2.8 to 5.3, and preferably from about 0.2% to 3% by weight of perfluoro(alkyl vinyl ether), said copolymer being polymerized and isolated in the absence of added alkali metal salts, having a melt flow rate of within the range of about 30±3 g/10 min, and having no more than about 50 unstable endgroups/10 6  carbon atoms can be extruded at high speed onto conductor over a broad polymer melt temperature range to give insulated wire of high quality.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a high melt flow copolymer oftetrafluoroethylene and hexafluoropropylene capable of being extruded athigh speed.

2. Description of Related Art

U.S. Pat. No. 5,677,404 discloses an improved fluoropolymer, wherein theimprovement enables the fluoropolymer to be extruded at high speedswithout sacrifice of stress crack resistance. This polymer issuccessfully extruded on to a conductor to make insulated wire of highquality (fewer than 10 sparks and 2 lumps/13 km of conductor coated) atspeeds in excess of 1900 ft/min (579 m/min). The UL 444 industrystandard for spark failures is no more than 15 spark failures per 45,000ft (13.7 km) of coated conductor. A spark failure indicates a fault inthe insulation. Industry prefers that no more than 10 spark failures bepresent/13.7 km of insulated conductor to insure acceptable insulatedconductor. An additional quality criterion desired by the industry isthat for the same length of coated conductor, the insulation should haveno more than 2 lumps/13.7 km. Lumps in the insulation interfere with theultimate use of the insulated conductor; e.g. twisting together to formtwisted pair conductors, pulling the insulated conductor through narrowopenings.

Speeds up to 2250 ft/min (686 m/min) can be easily achieved. Higherspeeds are possible but non-polymer specific limitations arise.Therefore production of good quality insulated conductor at line speedsof from about 1750 to 2250 ft/min (533 to 686 m/min) is consideredexcellent performance. However, it has been found that the temperatureof the molten polymer in extrusion must be closely controlled to achieveexcellent performance. Loss of control results in unacceptably highincidences of insulation faults such as sparks (points at which thepolymer inadequately coats the conductor) and lumps (regions ofirregular geometry of the insulation). It has further been found thatlot-to-lot variations in the fluoropolymer melt flow rate can upset theclose control of extrusion and require time-consuming and wastefuladjustments, during which time unsaleable product is made. Reduction influoropolymer melt flow rate variation would impose significant economicpenalties.

Further polymer improvement is needed to permit high speed extrusion,particularly for extrusion of fluoropolymer insulation with few or nosparks or lumps over a broader temperature range than is now possible.

BRIEF SUMMARY OF THE INVENTION

It has been found that a partially-crystalline copolymer comprisingtetrafluoroethylene with hexafluoropropylene in an amount correspondingto HFPI of from about 2.8 to 5.3, said copolymer being polymerized andisolated in the absence of added alkali metal salts, having a melt flowrate of within the range of about 30±3 g/10 min, and having no more thanabout 50 unstable endgroups/10⁶ carbon atoms can be extruded at highspeed onto conductor over a broad polymer melt temperature range to giveinsulated wire of high quality.

Another advantage of the copolymer of the present invention as will bedescribed in Example 1, is the ability of the copolymer to enjoy longextrusion runs without the need for shut down to clean the polymerextrusion tooling. This advantage is embodied in the process comprisingextruding the copolymer of described in the preceding paragraph, whereinthe copolymer is substantially free of alkali metal salt, at a melttemperature of at least about 740° F. (393° C.) and shear rate of atleast about 800 sec⁻¹.

DETAILED DESCRIPTION OF THE INVENTION

The fluoropolymers according to this invention are partiallycrystalline; i.e. they are not elastomers. They are copolymers oftetrafluoroethylene (TFE) and hexafluoropropylene (HFP). Copolymers arehere defined as polymers made by polymerizing two or more monomers. Thisincludes dipolymers of TFE and HFP wherein the HFP content of thecopolymer, characterized by an hexafluoropropylene Index (HFPI), isabout 2.0–5.3. The TFE/HFP copolymers of this invention also includepolymers comprised of TFE, HFP, and perfluoro(alkyl vinyl ether) (PAVE)wherein the alkyl group contains 1 to 5 carbon atoms. Examples of suchvinyl ethers include perfluoro(methyl, ethyl, and propyl vinyl ether)(PMVE, PEVE, and PPVE respectively). Typically the HFP content of thecopolymer will be characterized by an hexafluoropropylene Index (HFPI)of about 2.0–5.3. HFPI is the ratio of two infrared absorbances measuredon a film of the copolymer, which can be converted to wt % HFP bymultiplying by 3.2 as disclosed in the paragraph bridging cols. 3 and 4of U.S. Pat. No. 5,703,185. The TFE/HFP copolymer exhibits an MIT flexlife of at least about 1000 cycles, preferably at least about 2000cycles, and more preferably at least about 4000 cycles. Measurement ofMIT flex life is disclosed in U.S. Pat. No. 5,703,185. Generally theamount of PAVE monomer incorporated in the polymer according to thisinvention will be from about 0.2 to 3 wt %, based on the total weight ofthe copolymer. One preferred PAVE is perfluoro(propyl vinyl ether) andthe most preferred PAVE is perfluoro(ethyl vinyl ether). The melt flowrates (MFR) of FEP copolymers are determined in accordance with ASTMD1238. The MFR of polymers according to this invention are in the rangeof about 27 to 33 g/10 min, preferably about 28 to 32 g/10 min.

Polymerization is conducted in the absence of added alkali metal salts.The general procedure of Example 1 of U.S. Pat. No. 5,677,404 isfollowed. However, the initiator is made up with only ammoniumpersulfate. Potassium persulfate, a common alternative initiator orco-initiator with ammonium persulfate, is not used. It is also possibleto use organic initiators as disclosed in U.S. Pat. No. 5,182,342. Thewater for polymerization and washing is deionized. In theabove-mentioned Example 1, the copolymer is TFE/HFP/PEVE, though PPVE,PMVE, and other PAVE monomers, and combinations of these monomers, canbe substituted. MFR is controlled by the rate of addition of initiatorto the polymerization. After polymerization, the resulting polymerdispersion is coagulated by mechanical agitation. Coagulation may alsoby done by freezing and thawing, or by chemical addition. Acids orammonium salts may be used in chemical coagulation, but metal salts,particularly alkali metal salts may not. It is further preferred thatalkaline earth metal salts not be used in the process, for example ascoagulants, and that materials of construction of polymerization andprocessing equipment be chosen so that corrosion will not be a source ofmetal ions. The alkali metal ion content of the polymer is measured byx-ray fluorescence. For potassium as the analyte, the lower detectionlimit is 5 ppm in the polymer. Polymer according to this invention hasless than 50 ppm alkali metal ion, preferably less than about 25 ppm,more preferably less than about 10 ppm, and most preferably about lessthan about 5 ppm.

Polymers made using deionized water and polymerized and isolated withoutthe use of alkali metal salts are referred to herein as beingsubstantially salt-free.

It has been found that at high line speed in the conductor coatingoperation, the presence of alkali metal salt in the fluoropolymerpromotes the formation of fluoropolymer drool on the outer surface ofthe extrusion die and/or on the guider tip that is inside the die,through which the conductor passes, and this drool is periodicallycarried along the melt cone to the insulation on the conductor to appearas unacceptable lumps of insulation. This not the only source of lumps.Too high or too low polymer melt temperature can also cause lumps. Thepresence of alkali metal salt in the fluoropolymer contributes to thelump problem. The copolymer of the present invention is free of, i.e.does not contain, alkali metal salt in the sense that no alkali metalsalt is used in the polymerization or in the isolation of the resultingfluoropolymer.

The method of determination of alkali metal ion in the polymer can beillustrated by way of example of the determination of potassium ion. Theanalytical method is x-ray fluorescence (XRF). The XRF instrument isstandardized with polymer containing known amounts of potassium ion. Thezero ppm standard is made by polymerization in a potassium-ion freeenvironment and with a potassium-free recipe. For standards at otherconcentrations, the absolute values of potassium ion content aredetermined by proton induced x-ray emission (PIXE).

Polymers according to this invention are fluorinated as disclosed inU.S. Pat. No. 4,743,658 to convert thermally or hydrolytically unstableend groups to the stable —CF₃ endgroup. By thermally unstable is meantthat the endgroup reacts, usually by decomposition, at temperatures atwhich fluoropolymers are melt-processed, generally between 300 and 400°C. Examples of unstable endgroups affected by the fluorine treatment are—CF₂CH₂OH, —CONH₂, —COF, and —COOH. Fluorination is conducted so as toreduce the total number of the four types of unstable endgroups to nogreater than about 50/10⁶ carbon atoms in the polymer backbone.Preferably, the sum of these unstable endgroups after fluorine treatmentis no greater than about 20/10⁶ carbon atoms, and with respect to thefirst three-named endgroups, preferably less than about 6 suchendgroups/10⁶ carbon atoms. The fluorine treatment is followed by thesparging of the fluorine-treated pellets as disclosed in U.S. Pat. No.4,743,658, to rid the fluoropolymer of extractable fluoride.

The superiority of the polymer according to the present invention overthe lower MFR polymers exemplified in the prior art is seen in thebroader temperature range over which it can be extruded on to conductorto give high quality insulation. A further advantage of the polymer isits processibility at temperatures lower than polymers used in theseapplications heretofore.

EXAMPLES

A series of extrusion/melt draw-down processes are conducted using theextruder for melt draw-down extrusion coating of the copper conductor,all as described in Example 10 of U.S. Pat. No. 5,703,185. The linespeed is 2000 ft/min (610 m/min). The melt temperature of the copolymeris the temperature of the molten copolymer in the transition sectionbetween the extruder and the crosshead wherein the molten resin and theconductor are both traveling in the same direction. The melt temperatureis measured by a thermocouple contacting the melt. This is the generalprocedure used in the tests described hereinafter. 45,000 ft (13.7 km)lengths of fluoropolymer insulated copper conductor are produced, whichare then tested for sparks and lumps. The average of three runs (3×13.7km lengths) are used for each spark and lump determination. The testsfor sparks and lumps are conducted in-line on the insulated conductor.The spark test is carried out by exposing the outer surface of theinsulation to a voltage of 2.5 kV and recording spark failures. Lumpsare measured optically by laser measurement of changes in the diameterof the insulation. An increase in diameter of at least 50% is considereda lump. When spark failures exceed the quality limit, lump failures maynot be reported.

The composition of the copolymer of the Examples is like that of Example10: TFE/HFP/PEVE approximately 87/12/1 wt %. Melt flow is varied byvarying initiator feed during polymerization. This method is disclosedon p. 241 of Principles of Polymerization, 3^(rd) Ed, published by JohnWiley (1991), and in the sentence bridging cols. 3 and 4 of U.S. Pat.No. 6,103,844 and is the general method for changing MFR of thecopolymer in later Examples described herein.

Example A

The fluoropolymer is the copolymer described above having an MFR of 22g/10 min of the aforesaid Example 10, draw-down ratio of is 97:1, andthe melt temperature is 760° F. (404° C.). The insulated conductorexhibits 1 sparks and 0 lumps, acceptable quality.

Example B

Repetition of Example A, but decreasing the melt temperature to 757° F.(403° C.), results in the insulated conductor exhibiting greater than3.5 sparks. At 754° F. (401° C.) the insulated conductor exhibits 13.6sparks. When the melt temperature is further decreased to 750° F. (399°C.), the insulated conductor exhibits 38 sparks. When the melttemperature is further reduced to 740° F. (393° C.), the insulatedconductor exhibits 151 sparks. At 720° F. (382° C.) melt temperature,the insulated conductor exhibits 620 spark failures. The increase inlumps follows a similar pattern. Example B reveals the extremesensitivity of the extrusion/melt draw-down process to small changes inmelt temperature when polymer of this MFR is used.

Example C

Repetition of Example A, but increasing the melt temperature to 767° F.(408° C.), decreases the melt strength of the cone, leading toincreasing spark failures and increasing degradation of thefluoropolymer as indicated by the presence of black specs in theinsulation. The decrease in melt strength also periodically producescomplete rupture of the insulation. Shortening of the cone length helpsavoid rupturing, but the window of operation within which acceptablespark failures are obtained is only on the order of several ° C., whichis too narrow for commercial operation.

Example D

Repetition of Example A, but decreasing the draw-down ratio to 85:1,results in the insulated conductor exhibiting greater than 10 sparkfailures. The draw-down ratios used in the invention generally rangefrom about 60 to 120:1. The reduction from 97:1 to 85:1, which herecauses unacceptable quality, is too narrow a range for the level ofcommercial operability desired by the industry.

While high quality insulated conductor is producible using the copolymerof patent Example 10, Examples B–D show that the window of operatingconditions is quite narrow, making it difficult for differentmanufacturers to obtain the same desired result of both high quality andhigh line speed. As the line speed is reduced from 2000 ft/min (610m/min), the frequency of spark failures is reduced.

Example E

In this test, copolymer of increased MFR (26 g/10 min) is used. Withinthe draw-down ratio range of 60–100:1, the melt temperature range overwhich insulated conductor of acceptable quality can be produced at aline speed of 610 m/min is only 5° F. (2.8° C.), which is narrower thanthe variation in melt temperature typically present in the industry.Outside this narrow melt temperature range, either the spark failuresexceed 10 or the lumps exceed 2, or both.

Example F

In this test, the MFR of the copolymer is increased to 35 g/10 min and,using the melt temperature of Example A, the resultant insulatedconductor exhibits 20 spark failures and 20 lumps, inadequate quality.

Example G

In this test, the MFR of the copolymer is 30 g/10 min and using the melttemperature of Example A, the resultant insulated conductor exhibitsgreater than 10 sparks and greater than 2 lumps, thereby havinginadequate quality.

Example H (The Invention)

Example G is repeated except that the melt temperature is decreased to740° F. (393° C.). Surprisingly, the resultant insulated conductorpasses both the spark and lump tests, exhibiting 0–3 sparks and 0–1lumps in repeat testing. This same result is obtained when the melttemperature is changed within the range 734 to 746° F. (390 to 397° C.)and the draw down ratio is 80–100:1. The same result is obtained whenthe MFR of the copolymer is varied within the range of 28–32 g/10 min,except that the melt temperature range of operability shifts slightly,e.g. at the MFR of 32 g/10 min, the melt temperature of 748° F. (398°C.) provides acceptable quality. The same result is obtained when therange of draw-down ratios in narrowed to 60–100:1, except that the melttemperature window narrows to 7° C. These good results are obtained whenthe copolymer is pigmented white or orange and the cone length is in therange generally used in industry (25 to 75 mm). As the MFR or melttemperature moves out of these ranges, the occurrence of sparks andlumps increases drastically. The MFR range of 30±3 g/10 min and melttemperature range of 393° C.±6° C. include the transition fromacceptable quality to borderline quality, the narrower MFR and melttemperature ranges giving the most consistent highest quality results.These results are obtained when the line speed is varied from 533 m/minto 686 m/min and give the appearance of being obtainable at even higherline speeds, which were not tested because of the limits of practicalcontrollability in commercial operation. When the melt temperature isdecreased below 730° F. (388° C.), e.g. in the range of 720–729° F.(382–387° C.), or above 750° F. (399° C.), the quality of the insulatedconductor at the line speed of 610 m/min becomes unacceptable.

The results of Example H show that the polymer according to thisinvention can be extruded over an adequate range of operating conditionswith respect to melt temperature, range of draw ratios, cone length, anddifferences in pigmentation, for making high quality product inindustrial operation. The fact that the polymer melt temperatures arelower rather than higher than typical polymer melt temperatures isadvantageous because temperature-related polymer degradation is reduced.In addition the polymer according to the invention, being made andisolated in the absence of alkali metal salts, has increased thermalstability compared to alkali metal ion containing fluoropolymers.Furthermore, the polymer can be made within the product specificationsunder the normal operating conditions of fluoropolymer manufacture.

In the foregoing Examples, copolymers of different MFRs are tested overa range of melt temperatures typically varying by at least 30° F. andmore often 40° F. Within the range of 720° F. to 767° F. (382 to 408°C.), it has been discovered that a relatively narrow melt temperaturerange centered around 740° F. (393° C.) provides the acceptable result,with unacceptable results arising sharply outside the narrow melttemperature range.

The preferred copolymers of the present invention have an MFR in therange of about 30±3 g/10 min, are free of alkali metal salt, and havelow unstable endgroups as described above, when melt drawn at a melttemperature in the range of about 393° C.±6° C., through a broad rangeof draw down ratios such as 80–100:1, give wire insulation of highquality. More preferably, the MFR is in the range of about 30±2 g/10 minand said melt temperature is in the range of about 393° C.±4° C. and thedraw-down ratio can be in the range of about 60–120:1, and preferablythe extrusion/melt draw-down process is conducted wherein the operatingwindow to produce acceptable quality insulated conductor at a line speedof at least about 533 m/min is achieved within each of these ranges.

Example I

Another unexpected advantage of the copolymer of the present inventionis its improved extrudability under severe conditions of both a highmelt temperature and high shear rate. High temperature, e.g. at leastabout 740° F. (393° C.), exposes the copolymer to degradation. The sameis true of high shear, e.g. at least about 800 sec⁻¹, which causeslocalized overheating of the copolymer, also tending to causedegradation. The presence of alkali metal salt in the copolymer promotesthe degradation process, resulting in the plate out (deposit) ofdegraded copolymer on the die tooling, i.e. the die surface(s) incontact with the molten copolymer forming the outlet of the die. In thecase of coating (insulating) of a wire with the copolymer, the wireguide (guide tip), forms the inner surface of the tubular extrudate,whereby the inner surface of the die and the outer surface of the dietip form the tooling surfaces on which degraded copolymer deposits. Thisdeposit changes the size of the extrudate and forms a roughness on theouter surface of the extrudate (wire coating). This roughness lookssimilar to melt fracture, but is not curable by reducing the rate ofextrusion. This problem is aggravated by the presence of pigment in thecopolymer, present to provide color to the wire insulation. This problemis further aggravated by the presence of boron nitride foam cellnucleating agent as is typically present when the wire insulation is tobe foamed as it exits the extrusion die. The pigment and boron nitrideinteract with the plating of degraded copolymer on the die tooling toincrease the rate of plate out. When the effect of the plate out becomesnoticeable on the appearance of the extrudate or its change from desireddimension, the extrusion operation must be stopped for clean-out of dietooling. This results in lost production time and production ofexcessive scrap copolymer.

The stable end groups of the copolymer of the present invention tend toprevent degradation of the copolymer, but this is not sufficient whenthe extrusion is carried out both at high temperature and high shear.The absence of the alkali metal salt from the copolymer of the presentinvention importantly contributes to a greatly reduced rate of plateout, even when pigment and/or boron nitride are present in thecopolymer, such as in the following amounts 0.08 to 0.15 wt % pigmentand 0.5 to 0.8 wt % boron nitride, based on the total weight of thecopolymer, these amounts being typical for the function intended.

The shear rate to which the copolymer is subjected is a function of thesize of the die opening and the volumetric flow rate of the moltencopolymer through the die opening. The smaller the opening, the higherthe shear at a given flow rate. In the extrusion coating of wire withthe copolymer, the die opening is the annular orifice formed by theinner surface of the die and the outer surface of the die tip. Asdescribed in col. 9, l. 58–62 of U.S. Pat. No. 5,945,478, the shear rateis calculated from the equation 6q/(H²×πD), wherein q is the volumetricflow rate of molten FEP, H is the gap distance between the die (innersurface) and the die tip (outer surface), and D is the circumference ofthe gap at its midpoint (average diameter).

Some extrusion operations require a small gap, e.g. in the case ofextrusion foaming involving gas injection into the extruder to act asthe foaming agent at the outlet of the die, a small gap is necessary tobuild up sufficient pressure within the extruder to keep the gas foamingagent, e.g. nitrogen, dissolved in the molten polymer, so that foamingis delayed until extrusion from the die. A small gap results in a smalldraw down ratio (DDR). DDR is the ratio of the cross-sectional area ofthe annular die opening to the cross-sectional area of the finished wireinsulation. High speed extrusions, e.g. line speeds of 1500 to 2000ft/min (457 to 609 m/min) are carried out at high DDR, e.g. 80 to 100:1.The small gap required for extrusion of foamed insulation (foamed as theextrudate exits the die) requires a DDR of no greater than 25:1, therebyresulting in slower line speeds, e.g. 800 to 1200 ft/min (244 to 367m/min). In Example 10 of U.S. Pat. No. 5,703,185, referred tohereinbefore, the DDR to obtain a cone length of 2 in (5.1 cm) and linespeed of 457 m/min to 914 m/min was 97:1.

Using extrusion equipment similar to Example 10 of '185 for extrudingthe copolymer similar to Example H at a melt temperature of 740° F.(393° C.) and high shear rate (greater than 800 sec⁻¹) provides anextrusion result wherein no plate out is visible after continuousoperation for more than twice the time at which alkali metal saltcontaining polymer would be causing plate out. Thus productivity is morethan doubled using the copolymer of the present invention in the highshear (low DDR)/high temperature extrusion process.

In greater detail, the FEP copolymer used in this Example has an HFPI of3.8, less than 50 unstable end-groups, no detectable alkali metal saltcontent, and MFR 30 g/10 min. The copolymer (composition) also contains0.5 wt % boron nitride and 0.1 wt % TiO₂ pigment. The nitrogen pressurein the extruder is 3500 psi (24.13 MPa) and the DDR is 15:1 and linespeed is 1000 ft/min (305 m/min). The die tooling for obtaining this DDRto obtain wire insulation having an outer diameter of 0.034 in (0.09 cm)over an 0.0201 in (0.051 cm) diameter wire is as follows: 0.129 in (0.33cm) inner diameter of the die and 0.072 in (0.18 cm) outer diameter ofthe die tip. The shear rate to which the copolymer is subjected withthis tooling and volumetric flow rate of 22.95 lb/hr (10.42 kg/hr) is2764 sec⁻¹. When the tooling is changed to 0.166 in (0.42 cm) die innerdiameter and 0.093 in (0.24 cm) tip outer diameter to give a DDR of25:1, the shear rate decreases to 1308 sec⁻¹ at the same volumetric flowrate and line speed. As the DDR increases (gap increases) above 25:1,the foaming result becomes poorer because of premature foaming withinthe die tooling. To mathematically show the profound effect of DDR onshear rate, when the tooling is changed, i.e. the gap is increased, toprovide a DDR of 80:1 (die inner diameter of 0.297 in (0.75 cm) and dietip outer diameter of 0.167 in (0.42 cm), the shear rate drops to 230sec⁻¹ at the same volumetric flow rate and line speed. If the line speedis increased to 2000 ft/min (609 m/min) in the same tooling giving a DDRof 80:1, the shear rate increases to 460 sec⁻¹. The foaming process isinoperable at this high DDR.

Example J

Still another unexpected advantage of the copolymer of the presentinvention is the reduced dissipation factor of high speed datatransmission cable made using the copolymer of the present invention asinsulation covering the electrical conductor of the cable. Thus, thepartially-crystalline copolymer comprising TFE and HFP in an amountcorresponding to HFPI of from about 2.8 to 5.3, said copolymer beingsubstantially free of alkali metal salt, having a melt flow rate ofwithin the range of about 30±3 g/10 min, and having no more than about50 unstable endgroups/10⁶ carbon atoms, provides a surprising reductionin dissipation factor when used as the primary insulation of such highspeed cable. The absence of alkali metal salt from the copolymer isobtained, as described above, by carrying out the polymerizing andcopolymer isolation without using (adding) alkali metal salt in thepolymerization/isolation system. By “high speed” is meant that the datatransmission speed is at a frequency of least 10 GHz. EP 0 423 995 B1discloses in Table 1 that fluorinated TFE/PPVE copolymer exhibits adissipation factor which is better (lower) than that of fluorinatedTFE/HFP copolymer at 500 MHz frequency (0.000366 vs 0.000605,respectively). Since the higher dissipation factor results in reducedsignal strength, cable requiring the lower dissipation factor has usedthe more expensive TFE/PPVE copolymer and its subsequent improvedTFE/PAVE copolymers as the primary insulation. The TFE/HFP copolymer ofthe present invention exhibits a dissipation factor that is about asgood as TFE/PAVE copolymer at 500 MHz and this improvement carries overinto still higher speed cable transmitting date at a frequency of atlast 10 GHz, thus enabling the TFE/HFP copolymer of the presentinvention as described above to be used as the primary insulation(insulation covering the electrical conductor) in cable for thetransmission of data at a frequency of at least 10 GHz, said cablecomprising said electrical conductor and said insulation covering saidconductor, said cable exhibiting a dissipation factor at 10 GHz of nogreater than 0.00025. This cable is another embodiment of the presentinvention. Dissipation factor is measured on compression molded plaquesin accordance with ASTM D 2520 and dissipation factor is measured onthese plaques in accordance with the same ASTM procedure. It has beenfound that the dissipation factor determined by this procedure is areliable predictor of the dissipation factor of the cable. Thus, inaccordance with ASTM D 2520, the dissipation factor the molded plaque ofthe copolymer is considered to be the dissipation factor of the cable.The results of dissipation factors measurements is given in thefollowing table:

Dissipation factor Copolymer 10 GHz 15 GHz 20 GHz TFE/HFP copolymer0.00023 0.00018 0.00018 (Example I) TFE/PAVE copolymer 0.00022 0.000160.00018The TFE/PAVE copolymer in the table contains 3.3 wt % PPVE and has amelt flow rate of 5. It is fluorinated to have less than 50 unstable endgroups per 10⁶ carbon atoms. The dissipation factor of this copolymer isless than that of the fluorinated TFE/PPVE copolymer in Table 1 of EP 0423 995 B1 at 450 MHz i.e. 0.00035 vs. 0.000366 in '995 (ASTM D 150),indicating that the comparison in the above table is with a betterTFE/PAVE copolymer than that reported in the European patent, and yetthe TFE/HFP copolymer of the present invention exhibits a dissipationfactor that is just about as good as this better TFE/PAVE copolymer.Preferably, the dissipation factor of the cable at 15 GHz signaltransmission frequency is no greater than 0.00022 and more preferably,no greater than 0.00020.

The cable of the present invention preferably is that wherein thethickness of the copolymer insulation is less than 9 mils (0.23 mm), andmore preferably 6 to 8 mils (0.15 to 0.2 mm). The insulation may befoamed or unfoamed, i.e. solid. The foamed insulation may be present asthe primary insulation in twisted pairs of cables or in coaxial cables.The improved dissipation factor exhibited by the copolymer of thepresent invention and the cable incorporating this copolymer as primaryinsulation can also be described as a process invention, the processbeing for transmitting data at a frequency of at least 10 GHz by a cablecomprising an electrical conductor and insulation covering saidconductor, comprising forming said insulation from the copolymerdescribed above of and obtaining as a result thereof a dissipationfactor for said transmitting by said cable of no greater than 0.00025 at10 GHz.

1. A partially-crystalline copolymer comprising tetrafluoroethylene,hexafluoropropylene in an amount corresponding to a hexafluoropropyleneindex (HFPI) of from about 2.8 to 5.3, said copolymer being polymerizedand isolated in the absence of added alkali metal salt, having a meltflow rate of within the range of about 30±3 g/10 min, and having no morethan about 50 unstable endgroups/10⁶ carbon atoms.
 2. The copolymer ofclaim 1 having a melt flow rate within the range of about 30±2 g/10 min.3. The copolymer of claim 1 further comprising from about 0.2% to 3% byweight of perfluoro(alkyl vinyl ether).
 4. The copolymer of claim 3wherein said perfluoro(alkyl vinyl ether) is perfluoro(propyl vinylether).
 5. The copolymer of claim 3 wherein said perfluoro(alkyl vinylether) is perfluoro(ethyl vinyl ether).
 6. The copolymer of claim 1having no more than about 20 unstable endgroups/10⁶ carbon atoms.
 7. Thecopolymer of claim 1 wherein said alkali metal salt is a potassium salt.